How Climate Change is Changing Wildlife Disease

Climate change is real, and it’s happening right now all around us. We usually think of extreme weather, habitat loss and biodiversity decline - all of which are hugely important in their own right. But one major consequence is often overlooked: climate change is reshaping how diseases spread across the world, in both wildlife and humans.

So what do we mean when we mention wildlife disease?

Wildlife disease refers to any illness affecting a non-domesticated species. This includes infections caused by pathogens* such as viruses, parasites, bacteria and fungi, some of which can also have an impact on human health.

*Pathogens is just a fancy word for a disease causing microorganism.

As I discussed in my last article, we should no longer look at wildlife and human health in isolation. Climate change is further bridging this gap, increasing the connections between ecosystems, animals, and humans.

The Direct Effect: Weather & Pathogens

While increasing temperatures are commonly associated with climate change, the exact impact this will have on global disease dynamics is still not fully understood. Although research is being carried out on the effects of climate change on disease, only a small proportion of potentially zoonotic diseases have been studied in detail.

Current evidence suggests that, for many pathogens, increased temperatures can lead to higher transmission rates. However, it would be overly simplistic to attribute this solely to temperature, as there are multiple interacting factors at play, including how temperature affects different hosts and disease vectors.

For example, the plague (caused by the bacterium Yersinia pestis) is transmitted by fleas that live on rodent hosts. While the plague itself is not a result of climate change, warmer conditions can influence its dynamics by increasing rodent populations and accelerating flea reproduction, both of which can increase transmission risk.

However, this relationship is not linear - meaning it's not always as straightforward as increased temperature equals increased disease. At sufficiently high temperatures, the efficiency of transmission can actually decrease, highlighting just how complex these systems are. This demonstrates that disease responses to climate change are rarely straightforward, and often depend on multiple ecological and biological interactions occurring at once.

With climate change comes shifts in seasonal patterns. Weather patterns that were once predictable are becoming more variable and, in some cases, shifting significantly. This has important implications for disease, as many infections follow clear seasonal trends.

It’s not just temperature that drives these patterns. Rainfall and humidity also play a major role, often in more complex and less predictable ways. For example, increased rainfall and flooding can create ideal breeding conditions for species like mosquitoes, potentially increasing disease risk. At the same time, extreme flooding can wash away breeding sites, temporarily reducing their populations.

These interacting effects make it difficult to predict exactly how disease patterns will respond to changing weather conditions.

The indirect effect: changing animal movement

As temperatures shift and resources become less predictable, many species are changing their migration patterns, often moving towards cooler areas or regions with more abundant food sources. But it’s not just migration, even smaller changes in behaviour and daily movement can have important consequences for disease spread.

Let's look at polar bears as an example. As sea ice in the Arctic continues to melt, they are being forced to spend more time on land. This is having an impact on the species they are hunting. They are starting to feed more on migratory birds, species they wouldn’t normally interact with. And when new species come into contact, so do their pathogens, leading to greater disease spread.

This is creating new pathways for disease transmission between marine and terrestrial ecosystems - systems that are usually quite separate. Changes like this can ripple through entire ecosystems all the way down the food chain. When animal behaviour shifts, it changes what they eat, where they go and consequently what they’re exposed to. This means they are more likely to encounter unfamiliar pathogens that their immune systems may not be equipped to handle.

While these changes might seem subtle, they can reshape disease dynamics in ways that are difficult to predict, and even harder to control.

Vectors on the Move

One of the biggest, and probably most talked about, ways climate change is influencing disease is through vectors like mosquitoes and ticks. These species are incredibly sensitive to their environment. Temperature, humidity, rainfall, even small shifts can change where they’re populations are able to thrive or die. And as the climate warms, areas that were once too cold are suddenly becoming suitable habitats.

So what happens? They move.

And when they move, they don’t come alone. They bring the pathogens they carry with them. We're already seeing the impact of this with Mosquitos. Not only are warmer temperatures speeding up their breeding cycle but it also speeding up the reproduction of many of the pathogens they carry such as Dengue, Malaria, and Zika virus. It also means we're seeing the disease carrying species of mosquito move into new regions such as areas of Europe and North America.

Ticks, carriers of diseases like Lyme disease or Tick-Borne Encephalitis (TBE), are responding in a similar way. Increasing temperatures and more humid environments mean they are emerging earlier in the year and remain active for longer with higher survival rates.

However, again it's not as simple as 'increased temperature equals increased disease'. In some places, conditions may actually become too extreme. Too hot. Too dry. Not enough water for breeding. So while one region might see an increase, another might see a decline. This is why predicting disease spread changes due to climate change so challenging.

Stress, immunity, and wildlife health

Climate change doesn’t just affect where animals live, it affects how well they can cope.

Changing temperatures as well as altering food and water availability act as stressors. And that stress has a direct impact on the immune system. When animals are under constant environmental pressure, they have less energy available to invest in immune responses. Instead, that energy is redirected towards basic survival, staying cool, finding food and avoiding dehydration. Over time, this can lead to weakened immune function, making animals more susceptible to infection.

Changes in habitat can also reduce both the availability and quality of food. Poor nutrition limits the resources needed to maintain an effective immune system, further increasing vulnerability to disease. At the same time, climate change is altering where pathogens and parasites are found. As temperatures rise, many are able to expand into new regions, exposing wildlife to diseases they may have little or no resistance to.

Together, these pressures create a form of chronic stress. An animal might appear healthy on the surface, but physiologically it’s under strain. This means that even if exposure to a pathogen remains the same, the outcome of that infection can change. Infections may last longer, become more severe, and spread more easily through populations.

Ecosystem Disruption & Disease Spillover Risk

Climate change isn't happening in isolation. Habitat loss, overpopulation and changes in land use are all pressures that are reshaping ecosystems. As habitats shift or disappear, animals are forced to adapt. That often means moving into new areas or coming into closer contact with species they wouldn’t normally encounter, including livestock and humans, which can have serious consequences for disease transmission.

When species that don’t interact begin to overlap, pathogens gain new opportunities to spread. This is what we refer to as spillover - the transmission of a pathogen from one species into another. Often from a reservoir species into a new host, including humans.

Under stable conditions, ecosystems often act as a buffer, limiting these kinds of interactions. But as those systems become disrupted, those natural barriers begin to break down. The result is increased contact between wildlife and humans or livestock, creating more pathways for disease to move across species boundaries. This is why habitat loss combined with climate change is such a concern. It doesn’t just alter where species live, it changes how they interact. And as those interactions change, so too does the risk of disease emerging and spreading in new and unpredictable ways.

Why Does This Matter For People?

This is where everything starts to come together. It’s easy to think of wildlife disease, human health, and environmental change as separate issues. In reality, they’re all part of the same system, constantly interacting and influencing one another. This is the foundation of the One Health approach, not just recognising these connections, but understanding that they can’t be addressed in isolation. Wildlife health is directly connected to human health, and climate change is a shared driver shaping outcomes across both.

As climate change alters how wildlife are exposed to and respond to disease, it also has knock-on effects for human health. At the same time, human-driven pressures, from land use change to global movement, are influencing how animals and ecosystems function, further shaping patterns of disease spread, potentially leading to more zoonotic diseases.

Whichever way you look at it, humans, wildlife, and ecosystems are deeply interconnected. What might seem like a small, distant change can have wider consequences, often in ways that aren’t immediately obvious.

A Real-World Example

I recently came across a really interesting example of this while listening to the Wildlife Health Talks podcast by the Wildlife Disease Association, which discussed the potential spread of rabies to Australia. I decided to explore this further in the context of how climate change may influence disease emergence and movement.

Rabies is a viral disease that affects the central nervous system of mammals, including humans. It is transmitted through the saliva of infected animals, most commonly via bites. Globally, the vast majority of human cases are caused by dog bites, with rabies estimated to have caused around 60,000 human deaths each year.

Australia is currently rabies-free; however, there are concerns about the potential for reintroduction, particularly into northern regions. This risk is linked to the presence of rabies in nearby endemic regions, including parts of Indonesia, which reduces the geographical barrier between infected and disease-free areas.

From a climate change perspective, the relevance is indirect but important. Changes in temperature and habitat conditions can influence species distributions, potentially shifting where reservoir hosts and infected animals occur. At the same time, climate-driven environmental and economic pressures can alter human activity patterns, including maritime movement and fishing practices, which may increase the likelihood of long-distance animal movement between regions. Together, these ecological and human-driven processes shape potential pathways for disease incursion. This was highlighted in work by Emily G. Hudson, Victoria J. Brookes, and Michael P. Ward, who concluded that while the probability of rabies entering Australia is low, the potential consequences are significant enough to warrant ongoing surveillance and preparedness.

Climate change is influencing how diseases spread, and in some cases increasing risks. But it’s important to remember that this isn’t a simple, one-directional story. These systems are complex with multiple influences driving change. And that's the important thing to remember, the key takeaway of this isn't that everything is getting worse, it’s that things are changing, often in ways we’re still working to fully understand.

And that’s where the value of this research lies.

The more we understand how climate, wildlife and disease are connected, the better equipped we are to respond. Whether that’s improving surveillance, protecting ecosystems or making more informed decisions about how we interact with the environment, there are ways to reduce risk and manage these challenges.

So rather than panic, this is really about awareness. Understanding the bigger picture, recognising the connections and using that knowledge to make better decisions moving forward.

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